Whilst I remain hugely sceptical about the cleanliness of athletics, or even most sports, I think the occasional "out of the blue" performance is certainly possible. As mrcharly has said, Paula Radcliffe's improvements were incremental and she didn't suddenly become so fast. I think you can have a day when "it all just comes together". Unfortunately, for many, this may happen in training, rather than in an event. I have my own little example: before my kneee finally succumbed to an old rugby injury, I used to be a keen and reasonable, if unremarkable runner. I had a hilly training loop which used to take me about 36 minutes, depending on the conditions underfoot. I always ran it as hard as I could. One day, I'd had some particularly good news and, elated, I went out for my loop. I ran 3 minutes (nearly 9%) faster than ever before. I never got near that again.
I continue to believe that remarkable performances are possible, without assistance. I continue to believe that Paula is not the greatest distance runner there has been up to now.